Filament support structure having vibration suppressing means

ABSTRACT

A cathode structure for electron discharge tubes comprising parallel filament wires extending longitudinally in an axial plane of the tube with all wires carrying input current arranged as a group on one side of the center of the structure and all wires carrying output current arranged as a second group on the other side thereof, filament support rods having peripheral grooves at equally spaced intervals along the length thereof attached at one end to the filament wires and at the other end to respective coil springs, and thin metallic frames surrounding each group of filament support rods in planes perpendicular thereto such that the ends of opposing cantilever arms attached to the frames engage the peripheral grooves of the support rods to restrain lateral movement thereof while permitting longitudinal movement to allow the coil springs to compensate for thermal elongation and contraction of the filament wires.

United States Patent [72] Inventors Jacob A. Randmer Wilton, Conn.; George J. Agule, Sandgate, Vt. [211 Appl. No. 665,091 [22] Filed Sept. 1, 1967 [45] Patented Mar. 2, 1971 [73] Assignee The Machlett Laboratories, Incorporated Springdale, Conn.

[54] FILAMENT SUPPORT STRUCTURE HAVING VIBRATION SUPPRESSING MEANS 4 Claims, 10 Drawing Figs.

[52] U.S. Cl 313/278, 313/257, 313/269, 313/284, 313/285 [51] Int. Cl H01jl/94, HO 1 j 19/48 [50] Field ot'Search 313/284, 285, 278, 257, 269

[56] References Cited UNITED STATES PATENTS 2,494,853 1/1950 Alma 313/269X 2,742,587 4/1956 Armstrong 313/273 3,299,310 1/1967 Freggens 3,122,671 2/1964 Bennett Primary Examiner.lohn W. l-luckert Assistant Examiner-Barry Estrin Attorneys-Harold A. Murphy and Joseph D. Pannone arranged as a group on one side of the center of the structure and all wires carrying output current arranged as a second group on the other side thereof, filament support rods having peripheral grooves at equally spaced intervals along the length thereof attached at one end to the filament wires and at the other end to respective coil springs, and thin metallic frames surrounding each group of filament support rods in planes perpendicular thereto such that the ends of opposing cantilever arms attached to the frames engage the peripheral grooves of the support rods to restrain lateral movement thereof while permitting longitudinal movement to allow the coil springs to compensate for thermal elongation and contraction of the filament wires.

PATENTEDMAR 2m 3567.988

SHEET 2 [IF 3 II M 5& w 76\ 2 IN VENTORS JACOB A. RANDMER GEORGE J. AGULE FILAMENT SUPPORT STRUCTURE HAVING VIBRATION SUPPRESSING MEANS BACKGROUND OF THE INVENTION This invention relates to an electrode structure and is more particularly concerned with a cathode structure for electron discharge tubes.

In one type of power tube, the cathode, grid and anode electrodes are not concentric as in conventional power tubes but are arranged in parallel planes. In order to utilize the entire emitting surface of the cathode, it is located between spaced, parallel sections of the grid and further-spaced, parallel sections of the anode. Since each longitudinal half of the cathode is exposed to the grid and anode in this arrangement of electrodes, an electron current can be drawn from both sides of the cathode toward the grid and anode.

In prior art tubes of the presently described type, the cathode usually comprises a linear array of parallel filament wires extending longitudinally in an axial plane of the tube. Generally, the filament wires are attached at one end to a similar array of aligned support rods which, in turn, are attached to one of two supporting decks. Alternate filament support rods are attached to one support deck and the other support rods are attached to the other support deck. The two decks are insulated from each other and connect to respective cathode terminals of the tube. In some power tubes, the filament support rods pass insulatingly through aligned holes in the support decks and connect to respective coil springs on the far side. The coil springs maintain tension on the filament wires and compensate for the effects of thermal elongation or contraction when the filament wires are heating or cooling.

In order to confine electron emission from the directly heated filaments to the anode region of the tube and to locate the coil springs and support decks in a less-heated part of the tube, the intervening support rods are made as long as possiie. However, the long filament support rods have a serious disadvantage when the tube is subjected to shock or vibration. It has been discovered from such tests that filament wires tend to break at the point of attachment to the support rod. This is especially true when the support rods are only slidably engaged by holes in the support decks and connected to coil springs on the other side. The basic problem is to restrain lateral movement of the filament support rods but still permit longitudinal movement thereof so that the coil springs can compensate for thermal elongation or contraction of the attached filament wires.

The problem of restraining lateral motion of the filament support rods is complicated by the fact that adjacent the rods are usually at different electrical potentials in this type of power tube. Alternate filament support rods carry input current to the attached filament wires and the other support rods conduct output current from the attached filament wires. A single supporting device for restraining lateral motion of all the filament support rods would have to protect against the possibility of electrical shorts occurring between adjacent support rods. Two separate supporting devices to restrain lateral motion of alternate support rods are complex and costly. Dielectric bushings, tried as one solution to this problem, do restrain lateral motion while allowing longitudinal motion of the filament support rods. However, the resulting increase in friction during longitudinal motion of the filament support rods at high temperatures requires coil springs that will snap the filament wires at these elevated temperatures.

Another factor that should be given consideration in the solution of this problem is the requirement for uniform intereiectrode spacing between the filament wires and the adjacent, parallel sections of the grid. A supporting device that restrains lateral motion of the filament support rods should not distort the attached filament wires at the high temperatures encountered in power tube application. Any alternation in the precise alignment of the directly heated filament wires causes the electrical characteristics of the tube to vary unpredictably. In

fact, a supporting device that restrains lateral motion of the filament support rods should maintain the attached filament wires in accurate alignment with each other and with the grid.

SUMMARY OF THE INVENTION The present invention features a unique suspension and support system for the filament support rods. It will accurately align the attached filament wires, restrain lateral motion of the filament support rods without distortion and will allow longitudinal motion so that the coil springs attached to one end of the support rods can maintain a tension on the filament wires attached to the other end of the support rods. This is accomplished by a linear array of parallel filament wires which are arranged so that all wires connected to one cathode terminal are located at one side of the planar filament structure and all wires connected to the other cathode terminal are on the other side. With the filament wires segregated in this way, the adjacent wires in each group are at the same electrical potential, and a common, uninsulated supporting device can be used to restrain lateral movement in each group. The filament wires are attached to support rods which have peripheral grooves lying in selected planes. The rods pass through perpendicularly disposed, thin metallic frames and connect to respective coil springs on the far side. The thin metallic frames are supported in planes adjacent to the planes of the peripheral grooves and have opposing cantilever arms which fit into the peripheral grooves on the support rods. The cantilever arms restrain the support rods from moving laterally and flex to allow the support rods to move longitudinally. Thus, the coil springs can expand to take up any slack produced in the filament wires during thermal elongation or compress when the filament wire is cooling.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference is made to the drawings wherein:

FIG. 1 is an axial sectional view of a typical tube embodying this invention;

FIG. 2 is a fragmentary, cross-sectional view taken along line 2-2 of FIG. 1 looking in the direction of the arrows;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1 looking in the direction of the arrows;

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 1 looking in the direction of the arrows;

FIG. 5 is an enlarged view of a typical filament support rod;

FIG. 6 is an enlarged fragmentary view showing the mounting and construction of the insulated support box;

FIG. 7 is an enlarged fragmentary view showing the mounting and construction of the uninsulated support box;

FIG. 8 is an enlarged view of a typical metal frame having flexible cantilever supporting arms;

FIG. 9 is an enlarged fragmentary view, partially in section, showing the position of the cantilever supporting arms when the filaments are hot; and

FIG. 10 is an enlarged fragmentary view, partially in section, showing the position of the cantilever supporting arms when the filaments are cold.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, wherein like characters of reference designate like parts throughout the several views, the illustrative tube embodying the invention, as shown in FIG. 1 and FIG. 2, comprises a gastight envelope closed at one end by an oval-shaped anode cup 10 having extended flat sides 11 and 12. Anode 10 is made of a highly conductive metal, such as copper, and encloses a cavity designated generally as 13. The open end of anode 10 terminates in an outwardly extending flange 14 which has a circular perimeter and which functions as the anode terminal of the tube.

A metallic sleeve 15, preferably kovar, is hermetically attached, at one end, to flange 14 and circumferentially sealed at the opposite end to one end of a dielectric cylinder 16. The

other end of cylinder 16 is sealed, around its entire periphery, to the collar portion of a metallic member 20, preferably copper, which provides the grid terminal of the tube. Grid terminal member 20 extends radially inward of cylinder 16, flaring back toward anode cavity 13, and is attached, at the other end, to tubular support 21 by suitable means such as welding or brazing. A dielectric cylinder 40 is sealed throughout one end to the other side of grid terminal 20 and throughout the other end to collar portion of cathode terminal member 41. Extending radially inward and flaring back toward the anode within the tapering cavity formed by annular grid terminal 20, cathode terminal 41 is circumferentially attached, at the other end, to a circular support disc 42 preferably of copper. A cathode terminal 61 is insulatingly connected to cathode terminal 41 by an intervening dielectric cylinder 60 which is peripherally sealed at each end to one of the respective cathode terminals. Cathode terminal members 41 and 61 are advantageously formed of the same material as grid terminal 20 and provide the external connections to the cathode filaments. The gastight envelope is completed by cathode terminal 61 extending radially inward and flaring back toward the anode within the tapering cavity formed by annular, cathode terminal 41 and is hermetically attached, at the other end, to a second circular support disc 62, preferably of copper. An exhaust tubulation 63 is located in the center of support disc 62 is sealed off after processing and evacuation.

Mounted on the end of tubular support 21 remote from grid terminal 20, is a metallic disc 22 which has a centrally located slot 23, as shown in FIG. 3. Disc 22 is attached to tubular support 21 by suitable means such as screws 24. Spaced grid rods 25, having one end welded or brazed into respective holes located adjacent the long sides of slot 23, extend longitudinally of the tube and into the anode cavity 13. The opposite ends of grid rods 25 are attached to an oblong plate 26 (FIG. 2) by welding or brazing them into respective holes located adjacent the long sides of a slot 27, thus maintaining the grid rods 25 in the desired spaced relationship. As a result of this construction, the grid comprises two planar sections, indicated generally as 28 and 29, which are parallel to each other. Each planar section, for example, 28, is made up of a linear array of parallel rods 25 which are located on the same side of slots 23 and 27. The central portion of slots 27 is enlarged to form a circular aperture 30 that permits plate 26 to slidingly engage a dielectric bushing 31 affixed to a center mast 32. Center mast 32 is attached to the center of a circular support deck 39, as by journaling or brazing, and is located on the longitudinal centerline of the tube. Thus, lateral motion of the grid is restricted, thereby maintaining uniform spacing between planar sections 28 and 29 of the grid and the flat extended portions 11 and 12 of anode 10.

A capnut 33, (FIG. 1) journaled onto the threaded end of center mast 32, forces the shoulder of dielectric bushing 31 against a metallic crossarm 34. Crossarm 34 is provided with a center bore for slidingly engaging the shank portion of dielectric bushing 31 in one end and the shank portion of a dielectric bushing 36 in the other end. Thus, crossarm 34 is insulatingly mounted on center mast 32. The pressure exerted by capnut 33 on crossarm 34 forces the periphery of bore 35 against the flange of bushing 36 and presses it against a cap washer 37. Cap washer 37, in turn, bears against one end of a metallic sleeve 38 which surrounds center mast 32, and forces the other end of sleeve 38 against a support deck 39. The central portion of slot 23 in metal disc 22 is enlarged to allow center mast 32, with surrounding metal sleeve 38, to extend through the center thereof in spaced relationship.

As shown more clearly in FIG. 4, the side of support deck 39 opposite the center mast structure is mounted, by conventional means, such as brazing, on the ends of four spaced posts 62a, preferably copper, which are perpendicularly disposed in support deck 39. Posts 62a are symmetrically located and extend insulatingly through aligned holes in support disc 42 and are attached, at the other end, to support disc 62. Thus a continuous conductive path from cathode terminal 61 to support deck 39 is provided by metal support disc 62 and metallic support posts 62a. A metallic post 43, preferably copper, is perpendicularly disposed to support disc 42 and attached to the center thereof by conventional means, such as brazing. Thus post 43 forms part of a conductive path which includes metal support disc 42 and terminates at cathode terminal 41.

Filament support rods 48-50 and 6870 are identical in configuration to the typical support rod shown in FIG. 5. Each support rod is provided with a ,cavity end 80, a reduced diameter portion 81 at the opposite end, which thereby forms shoulder 82 at the junction with the normal diameter of the rod, and spaced, peripheral grooves 83 and 84 between the ends. Referring to FIG. 6 and FIG. 7, a helical spring 85 is placed over the reduced diameter end 81 of rods 48-50 and 68-70. Each helical spring 85 is supported by a cap washer 86 which encircles the reduced diameter 81 and bears against shoulder 82 of .rods 48-50 and 68-70. Copper straps 44- 46 engage the reduced diameter portion 8-1 of respective support rods 48-50. Copper straps 64-66 are similarly affixed to the reduced diameter portions 81 of support rods 68- -70 respectively. A cap 87 is pressed onto the reduced diameter portion 81 of each rod 4850 and 68-70 and crimped or welded in place. The cavity end 80 of each rod 48- 50 is inserted into the bore of respective dielectric bushings 51 which are embedded in support deck 39 and pushed longitudinally through a support box 52 until engaged and supported in a manner to be described. The cavity end 80 of each rod 687 0 is inserted into a respective spaced hole 71 in support deck 39 and pushed longitudinally through a support box 72 until engaged and supported in a manner to be described. The other ends of copper straps 44-46 are secured to the adjacent end of center post 43 (FIG. 1) by suitable means, such as screw 47. The free ends of the other copper straps 64 66 are attached to support deck 39 (FIG. 4) by conventional means, such as screws 67.

Support boxes 52 and 72 are identical in construction and configuration. Each box is four-sided having either closed or open ends in the planes parallel to the plane of the drawing. L- shaped mounting brackets 91 and 92 are attached to vertical sidewalls 93 and 94 respectively by suitable means, such as welded or brazing. Opposing sidewalls 93 and 94 are identical in construction, each being formed from blanks of sheet metal by bending opposite edges of each blank to form supporting flanges 95 and 96 respectively. Identical metal plates 97 and 98 are attached to the supporting flanges 95 and 96 at opposite ends of the opposing vertical sidewalls 93 and 94 to complete the four-sided box. As shown in FIG. 8, metal plates 97 and 98 each comprise a square of sheet metal having H shaped slots 99 stamped therein to form opposing cantilever arms 100 and 101, and intervening crossmembers 102 and 103 attached to the resulting sheet metal frame 104. An arcuate recess 105 in the free end of each cantilever arm forms part of a circular aperture 106 therebetween. Support box 52 is insulatingly mounted on support deck 39 by means of dielectric bushings 56 and dielectric washers 55. Screws 53 journaled intosupport deck 39 press against respective metal washer 54 thereby forcing respective dielectric washers 55 against the flange portion of respective L-shaped brackets 91 and 92. Washers 55 and the flange portions of brackets 91 and 92 encircle a shank portion of bushing 56. The flange portions of the brackets 91 and 92 bear against shoulders on respective bushings 56, which, in turn, are seated upon support deck 39. Support box 72 is conductively mounted on support deck 39 by means of metallic spacers 75. Screws 73 journaled into support deck 39 press respective metal washers 74 against the flange portion of respective L-shaped brackets 91 and 92 which, in turn, are forced against one end of respective metal spacers 75. The other end of spacers '75 bear against support deck 39.

As support rods 48, 49 and 50 are pushed through the center bore of respective dielectric bushings 51, the cavity ends 80 of respective rods 48, 49 and 50 protrude into aligned apertures 106 between respective opposing cantilever arms 100 and 101 of metal plate 98. A continued pressure on support rods 48-50 in the longitudinal direction causes the flexible cantilever arms 100 and 101 to bend in that direction, and the arcuate indentations 105 in the free ends of cantilever arms 100 and 101 slidingly engage portions of the circumference of rods 48-50. The cavity end 80 of respective support rods 48-50 is urged toward crossarm 34 (FIG. 1) until the free ends of opposing arms 100 and 101 of plate 97 snap into peripheral grooves 83 and those of plate 98 snap into peripheral grooves 84 of the respective support rods 48, 49 and 50. In this position, grooves 83 and interacting cantilever arms 100 and 101 are aligned coplanar with the frame 104 of plate 97; and grooves 84 and interacting cantilever arms 100 and 101 are aligned coplanar with the frame 104 of plate 98, as shown in FIG. 9. When the support rods 48-50 are interacting cantilever arms 100 and 101 are thus disposed, the periphery of the arcuate indentations 105 in opposing ends of cantilever arms 100 and 101 bear tightly against the rods at the base of the grooves 83 and 84, thus holding rods 48-50 firmly in place. Crossmembers 102 and 103 attached to metal frame 104 respective plates 97 and 98 extend between adjacent support rods 48-59 and 49-50. Thus, metal frames 104 of respective plates 97 and 98 surround the entire group of support rods 48-50, and crossmember 102 and 103 cooperate with frames 104 to form smaller frames which surround each of the respective support rods 48-50. Similarly as support rods 68, 69 and 70 are pushed through respective holes 71 in support deck 39, peripheral grooves 83 and 84 of the respective support rods are engaged by opposing cantilever arms 100 and 101 of respective metal plates 97 and 98 in support box 72. When thus engaged, grooves 83 of respective rods 68-70 and the interlocking cantilever arms 100 and 101 are disposed in the plane of plate 98; and grooves 84 of respective support rods 68-70 and the interlocking cantilever arms 100 and 101 are disposed in the plane of plate 98, as shown in FIG. 9. In this position, the periphery of arcuate indentations 105 in the opposing free ends of cantilever arms 100 and 101 bear tightly against the rods at the base of the grooves 83 and 84 of respective rods 68, 69 and 70. Metal frames 104 of respective plates 97 and 98 surround the entire group of support rods 68-70 and crossmembers 102 and 103 cooperate with frames 104 to form smaller frames which surround each of the respective support rods 68-70. The support rods 48-50 and 68-70 extend longitudinally through slot 23 (FIG. 3) in metal disc 22, equidistant from grid rods 25 which are attached to the long sides of slot 23.

One end of filament wires 57, 58 and 59 are inserted into respective holes in one side of crossarm 34 (FIGS. 1 and 2) and are attached thereto by convenient means, such as welding or brazing. One end of another set of filament wires 76, 77, and 78 are similarily secured into respective holes in the other side of crossarm 34. The filament wires extend longitudinally in two groups, wires 57-59 on one side of center mast 32 and wires 76-78 on the other side. Filament wires 57-59 and 76- -78 are cut to the desired length. With helical springs 85 compressed, the other ends of filament wires 57, 58 and 59 are inserted into cavities 80 of respective support rods 48, 49 and 50 and attached therein by suitable means, such as welding. The other ends of filament wires 76, 77 and 78 are similarly secured into cavities 80 of support rods 68, 69 and 70 while the attached helical springs 85 are compressed. With release of helical springs 85, the cathode structure is complete, with springs 85 maintaining the filaments taut. Filament wires 57- -59, and 76-78 are symmetrically located between grid rods 25 of planar grid sections 28 and 29 (FIG. 2) such that a potential applied to grid rods 25 will control the flow of electrons from heated filaments 57-59 and 76-78 to the flat portions 11 and 12 of the anode 10.

When the helical springs 85, affixed to support rods 48-50 and 68-70 are compressed, the respective support rods are moved longitudinally toward crossarm 34, and peripheral grooves 83 and 84 of the respective support rods are no longer disposed in respective planes of plates 97 and 98, as shown in FIG. 10. As grooves 83 and 84 move out of the planes of respective plate 97 and 98, a shoulder of each groove bears against the opposing ends of interacting cantilever arms 100 and 101 and bends the flexible arms in the direction of movement. Since one end of each cantilever arm is fixedly attached to frame 104 of the respective plates97 and 98, the opposing free ends of cantilever arms 100 and 101 travel in an arc, moving slightly away from the bases of grooves 83 and 84. The resilient cantilever arms 100 and 101 tend to return to the plane of the attached metal plate 97-and 98, respectively, and thus cooperate with the helical springs in maintaining tension in the longitudinal direction on the engaged support rods 48-50, 68-70 and the attached filament wires 57-59 and 76-78. Although the opposing ends of cantilever arms and 101 are moved slightly away from the bases of the respective grooves 83 and 84, the arcuate indentations in the ends of arms 100 and 101 still engage a portion of the periphery of the rod within the grooves 83 and 84 and maintain a spring tension against the contacting shoulder of the grooves. Thus, filament support rods 48-50 and 68-70 are restrained from moving in a lateral direction by the interlocking ends of the opposing cantilever arms 100 and 101. However, rods 48-50 and 68-70 may move in the longitudinal direction by flexing the resilient cantilever arms 100 and 101, as described above.

In the operation of the tube, a filament current heats the filament wires 57-59 and 76-78 to an incandescent state, thus producing a flow of electrons to the anode 10. When heating up to this condition, filament wires 57-59 and 76- -78 are subject to thermal elongation in the longitudinal direction which is compensated for by helical springs 85 expanding in the longitudinal direction. The resulting longitudinal motion of filament support rods 48-50 and 68-70 carries peripheral grooves 83 and 84 with interlocking cantilever arms 100 and 101 back into the plane of respective metal plates 97 and 98, as shown in FIG. 9. As grooves 83 and 84 move back toward this position, the opposing free ends or arms 100 and 101, travelling in an arc, close in on the bases of the grooves 83 and 84. Thus, when the filaments are hottest and consequently weakest, the arcuate indentations 105 in the opposing free ends of respective cantilever arms 100 and 101 are bearing tightly against the bases of grooves 83 and 84in support rods 48- 50 and 68-70.

The foregoing description discloses an invention that restricts lateral motion of filament support rods while allowing longitudinal motion thereof to permit auxiliary coil springs to remove slack from the attached filament wires and thereby compensate for thermal elongation of the wires and thereby compensate for thermal elongation of the wires during operation of the tube. This invention, as embodied in the support boxes described herein, features a novel suspension and supporting device for filamentary cathodes that minimizes the effects due to shock, vibration and friction. Because of the builtin accuracy of this supporting device, little operator skill is required to assemble a precisely spaced and aligned electrode structure. Production of the illustrative tube is thereby simplified and facilitated. This supporting device may also be adapted for use on other electrodes. For example, this device may be used to support and align the grid rods.

Although each metal plate 97 and 98 is shown with three pairs of cantilever arms, it is obvious that a greater or lesser number of opposing cantilever arms may be provided with slight modification of the metal plate. It is also possible to provide a stronger metal frame 104 without the cooperating cross member 102 and 103 for added rigidity. Thus, only the opposing cantilever arms 100 and 101 would extend into the opening surrounded by the stronger metal frame. Furthermore, the support rods may be provided with additional peripheral grooves so that additional metal plates like 97 and 98 may be used to restrain the lateral movement of the support rods. Alternatively, the peripheral grooves may take some other configuration, for example radial slots, and the indentations in the free ends of the cantilever arms may take some other contour than the arcuate one shown herein, for example the indentation may be V-shaped. Furthermore, the members that restrain lateral movement of the support rods while allowing longitudinal movement thereof need not be in the form of resilient cantilever arms to remain within the spirit and scope of this invention, as disclosed herein. These and other modifications which may occur to those skilled in the art are intended to be included in the appended claims.

We claim:

1. A cathode structure for an electron discharge tube comprising:

a linear array of parallel filament wires extending longitudinally in an axial plane ofzthe tube, and segregated into two groups one on each side of the central axis;

similarly aligned and segregated support rods having indentations in the outer diameter at equally spaced intervals along the longitudinal length of each rod and having one end attached to respective filament wires;

coil springs located adjacent the other ends of the support rods and connected thereto;

frame members surrounding each segregated group or support rods; and

pairs of aligned, resilient cantilever arms having ends attached to a frame member and opposing ends nesting in the indentations of respective support rods thereby restraining lateral movement but allowing longitudinal movement of the support rods.

2. A cathode structure as set forth in claim 1 wherein the indentations in the outer diameter of the support rods are circumferential grooves.

3, A cathode structure as set forth in claim 1 wherein said opposing ends of the cantilever arms are arcuate and partially encircle said indented portions of the respective support rods.

4. A cathode structure as set forth in claim 1 wherein said opposing ends of the cantilever arms extend to the full depth of said indentations at elevated temperatures. 

1. A cathode structure for an electron discharge tube comprising: a linear array of parallel filament wires extending longitudinally in an axial plane of the tube, and segregated into two groups one on each side of the central axis; similarly aligned and segregated support rods having indentations in the outer diameter at equally spaced intervals along the longitudinal length of each roD and having one end attached to respective filament wires; coil springs located adjacent the other ends of the support rods and connected thereto; frame members surrounding each segregated group or support rods; and pairs of aligned, resilient cantilever arms having ends attached to a frame member and opposing ends nesting in the indentations of respective support rods thereby restraining lateral movement but allowing longitudinal movement of the support rods.
 2. A cathode structure as set forth in claim 1 wherein the indentations in the outer diameter of the support rods are circumferential grooves.
 3. A cathode structure as set forth in claim 1 wherein said opposing ends of the cantilever arms are arcuate and partially encircle said indented portions of the respective support rods.
 4. A cathode structure as set forth in claim 1 wherein said opposing ends of the cantilever arms extend to the full depth of said indentations at elevated temperatures. 